1. Field of the invention
This invention relates to an optical memory device that conducts a recording operation, a reading operation, or an erasing operation of information by means of light beams, and to an optical memory apparatus with the said optical memory device.
2. Description of the prior art
In recent years, optical memory devices have come to public notice as a high density and high capacity memory device, which conducts a recording operation, a reading operation, or an erasing operation of information by means of light beams from semiconductor lasers or the like. An optical element (i.e., an optical pickup) that is composed of the above-mentioned semiconductor lasers, optical lenses for converging light beams from the semiconductor lasers so as to illuminate the optical memory device, photodetectors for detecting light reflected by the optical memory device, and so on must be moved at a high speed in a two-dimensional manner with regard to the said optical memory device, which causes difficulties in the positioning of the said optical element with high accuracy.
To overcome this problem, the above-mentioned optical memory device is generally formed into a disk, which is moved in one-dimensional manner in the radial direction thereof while the disk is turning around its axis, thereby attaining a recording operation, a reading operation, or an erasing operation of information on the disk.
Such a disk-shaped optical memory device comprises, as shown in FIG. 5, a disk-shaped substrate 13 that has a plurality of guide grooves 12 on one surface thereof, and a recording layer 14 that is disposed on the groove-side substrate 12. The other surface of the substrate 13 is irradiated with light beams 15 from a semiconductor laser (not shown) so as to achieve a recording operation, a reading operation, or an erasing operation of information. The optical memory device with the above-mentioned structure is advantageous in that the optical element can be arranged only on one side of the optical memory disk, which results in a miniaturized and lightweight optical element and which makes easy the movement of the optical element in the radial direction of the optical memory disk.
FIG. 6 shows another conventional diskshaped optical memory device, which is produced by sandwiching an adhesive layer 16 between a pair of optical memory disks of FIG. 5 in such a manner that the recording film 14-side surface of one disk faces that of the other disk through the adhesive layer 16. This optical memory device is advantageous in that both outer surfaces of the disks are irradiated with light beams 15, and accordingly the amount of information to be recorded is doubled. However, it is difficult for operators to recognize which is the front surface or the back surface of the optical memory disk at the time of the application of the disk to an optical memory apparatus. If the front surface (or the back surface of the disk is wrongly recognized to be the back surface (or the front surface) thereof, the disk will have to be removed from the optical memory apparatus and again installed into the apparatus so that the correct surface of the disk can face the optical element. The same problem arises in the optical memory device shown in FIG. 5 because the front surface of the disk is indistinguishable from the back surface of the disk.
To overcome this problem, optical pickups are disposed on both the front surface and the back surface of the disk, respectively. These optical pickups are very expensive, and moreover they make the optical memory apparatus large.